Wireless communication systems comprise a plurality of base stations (cell sites) and a plurality of mobile telephones (mobile units). The base stations are distributed geographically to define a plurality of cells. A cell is a geographical area representing the signal coverage of a cell site or a base station.
The base stations are connected to the public switched telephone network (PSTN) via a mobile telephone switch office (MTSO) using wire-line connections. The base stations communicate with the mobile units over wireless links or communication channels. As mobile units move among the cells, the base stations transfer the wireless links with the mobile units to other base stations in a procedure called handoff. An important objective of handoff technology is to accomplish a transfer of a conversation in progress without interruption (a soft handoff) or with minimal interruption (a hard handoff). To accomplish a soft handoff it is necessary for a mobile signal to be received by more than one base station.
Currently there are several types of wireless telecommunication systems in the world based on different modulation/demodulation technologies. The Code Division Multiple Access (CDMA), based on the IS95/IS2000 air interface standard, and the UMTS (Universal Mobile Telecommunication System) are among them. The CDMA and the UMTS systems are principally based on the direct sequence (DS) spread spectrum modulation/demodulation technology.
Both base stations and mobile units operating under DS spread spectrum modulation/demodulation technologies use a set of receivers called rake receivers to receive signals. A rake receiver employs a receiver technique which uses several baseband correlators (also called “fingers”) to individually process several multi-path components of a received signal. The correlator outputs are combined, called rake combining, to achieve improved communications reliability and performance. Using a rake receiver and the rake combining method, the base station demodulates a number of multi-path components of mobile transmitted signals to accomplish multi-path reception.
RTD is the time it takes for a signal to travel from the base station to the mobile unit and back to the base station, or vice versa. Each finger of the base station's rake receiver measures a RTD for the multi-path component of the mobile transmitted signal which the finger demodulates. The RTD indicator associated with each finger is dependent on that multi-path component and it will be unique among other assigned fingers' RTD indicators.
Along with recording the RTD indicators, each base station measures a set of RTD quality parameters, such as TIMESTAMP and ENERGY values, for an RTD indicator. The TIMESTAMP indicates the time at which the RTD indicator was collected at the base station. The ENERGY value indicates the energy level of a signal received at a finger in a rake receiver at a base station. These quality parameters form the RTD quality metric. The quality metric indicates the quality (accuracy) of the RTD indicator.
The operation of a wireless telecommunication system may require determining the geographic location (geo-location) of a specific mobile unit, e.g. for emergency 911 service. Geo-location of a mobile unit may be obtained using algorithms such as AFLT (Advanced Forward Link Trilateration) and EFLT (Enhanced Forward Link Trilateration). Geo-location accuracy using these algorithms is dependent upon obtaining accurate round trip delay (RTD) indicators.